Publish UMLBoard on Steam

Electron apps can be published also on Steam, deployment also does not look to complicated, this is a very short introduction:

How to package your web project for platforms like Steam

It should also be checked if we need the Steam API or whether it's enough to just deploy the app via steam:

How to integrate an HTML5 Electron based game with the Steam API.

Also check if the app can be deployed for all three platforms and whether the user license can be shared between platforms.